mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-26 11:41:09 +00:00
135 lines
5.5 KiB
Markdown
135 lines
5.5 KiB
Markdown
# GstBuffer
|
||
|
||
This document describes the design for buffers.
|
||
|
||
A `GstBuffer` is the object that is passed from an upstream element to a
|
||
downstream element and contains memory and metadata information.
|
||
|
||
## Requirements
|
||
|
||
- It must be fast
|
||
- allocation, free, low fragmentation
|
||
- Must be able to attach multiple memory blocks to the buffer
|
||
- Must be able to attach arbitrary metadata to buffers
|
||
- efficient handling of subbuffer, copy, span, trim
|
||
|
||
## Lifecycle
|
||
|
||
`GstMemory` extends from `GstMiniObject` and therefore uses its lifecycle
|
||
management (See [miniobject](additional/design/miniobject.md)).
|
||
|
||
## Writability
|
||
|
||
When a `GstBuffer` is writable as returned by `gst_buffer_is_writable()`:
|
||
|
||
- metadata can be added/removed and the metadata can be changed
|
||
|
||
- `GstMemory` blocks can be added/removed
|
||
|
||
The individual memory blocks have their own locking and READONLY flags
|
||
that might influence their writability.
|
||
|
||
Buffers can be made writable with `gst_buffer_make_writable()`. This
|
||
will copy the buffer with the metadata and will ref the memory in the
|
||
buffer. This means that the memory is not automatically copied when
|
||
copying buffers.
|
||
|
||
# Managing GstMemory
|
||
|
||
A `GstBuffer` contains an array of pointers to `GstMemory` objects.
|
||
|
||
When the buffer is writable, `gst_buffer_insert_memory()` can be used
|
||
to add a new `GstMemory` object to the buffer. When the array of memory is
|
||
full, memory will be merged to make room for the new memory object.
|
||
|
||
`gst_buffer_n_memory()` is used to get the amount of memory blocks on
|
||
the `GstBuffer`.
|
||
|
||
With `gst_buffer_peek_memory()`, memory can be retrieved from the
|
||
memory array. The desired access pattern for the memory block should be
|
||
specified so that appropriate checks can be made and, in case of
|
||
`GST_MAP_WRITE`, a writable copy can be constructed when needed.
|
||
|
||
`gst_buffer_remove_memory_range()` and `gst_buffer_remove_memory()`
|
||
can be used to remove memory from the `GstBuffer`.
|
||
|
||
# Subbuffers
|
||
|
||
Subbuffers are made by copying only a region of the memory blocks and
|
||
copying all of the metadata.
|
||
|
||
# Span
|
||
|
||
Spanning will merge together the data of 2 buffers into a new buffer
|
||
|
||
# Data access
|
||
|
||
Accessing the data of the buffer can happen by retrieving the individual
|
||
`GstMemory` objects in the `GstBuffer` or by using the `gst_buffer_map()` and
|
||
`gst_buffer_unmap()` functions.
|
||
|
||
The `_map()` and `_unmap()` functions will always return the memory of all
|
||
blocks as one large contiguous region. Using these functions might be more
|
||
convenient than accessing the individual memory blocks at the expense of
|
||
being more expensive because it might perform memcpy operations.
|
||
|
||
For buffers with only one `GstMemory` object (the most common case), `_map()`
|
||
and `_unmap()` have no performance penalty at all.
|
||
|
||
- **Read access with 1 memory block**: The memory block is accessed and mapped
|
||
for read access. The memory block is unmapped after usage
|
||
|
||
- **write access with 1 memory block**: The buffer should be writable or this
|
||
operation will fail. The memory block is accessed. If the memory block is
|
||
readonly, a copy is made and the original memory block is replaced with this
|
||
copy. Then the memory block is mapped in write mode and unmapped after usage.
|
||
|
||
- **Read access with multiple memory blocks**: The memory blocks are combined
|
||
into one large memory block. If the buffer is writable, the memory blocks are
|
||
replaced with this new combined block. If the buffer is not writable, the
|
||
memory is returned as is. The memory block is then mapped in read mode.
|
||
When the memory is unmapped after usage and the buffer has multiple memory
|
||
blocks, this means that the map operation was not able to store the combined
|
||
buffer and it thus returned memory that should be freed. Otherwise, the memory
|
||
is unmapped.
|
||
|
||
- **Write access with multiple memory blocks**: The buffer should be writable
|
||
or the operation fails. The memory blocks are combined into one large memory
|
||
block and the existing blocks are replaced with this new block. The memory is
|
||
then mapped in write mode and unmapped after usage.
|
||
|
||
# Use cases
|
||
|
||
## Generating RTP packets from h264 video
|
||
|
||
We receive as input a `GstBuffer` with an encoded h264 image and we need
|
||
to create RTP packets containing this h264 data as the payload. We
|
||
typically need to fragment the h264 data into multiple packets, each
|
||
with their own RTP and payload specific header.
|
||
|
||
```
|
||
+-------+-------+---------------------------+--------+
|
||
input H264 buffer: | NALU1 | NALU2 | ..... | NALUx |
|
||
+-------+-------+---------------------------+--------+
|
||
|
|
||
V
|
||
array of +-+ +-------+ +-+ +-------+ +-+ +-------+
|
||
output buffers: | | | NALU1 | | | | NALU2 | .... | | | NALUx |
|
||
+-+ +-------+ +-+ +-------+ +-+ +-------+
|
||
: : : :
|
||
\-----------/ \-----------/
|
||
buffer 1 buffer 2
|
||
```
|
||
|
||
The output buffer array consists of x buffers consisting of an RTP
|
||
payload header and a subbuffer of the original input H264 buffer. Since
|
||
the rtp headers and the h264 data don’t need to be contiguous in memory,
|
||
they are added to the buffer as separate `GstMemory` blocks and we can
|
||
avoid to memcpy the h264 data into contiguous memory.
|
||
|
||
A typical udpsink will then use something like sendmsg to send the
|
||
memory regions on the network inside one UDP packet. This will further
|
||
avoid having to memcpy data into contiguous memory.
|
||
|
||
Using bufferlists, the complete array of output buffers can be pushed in
|
||
one operation to the peer element.
|